1. Field of Invention
The invention relates in general to the use of radio frequency identification (RFID) tags and RFID tag readers (also known as “interrogators”). In particular, the invention relates to the configuration and operation of RFID tags that can self tune an integrated antenna so as to automatically adjust to a resonant frequency.
2. Related Art
Radio frequency identification (RFID) tags are electronic devices that may be affixed to items whose presence is to be detected and/or monitored. RFID tags are classified based on standards defined by national and international standards bodies (e.g., EPCGlobal and ISO). Standard tag classes include Class 0, Class 1, and Class 1 Generation 2 (referred to herein as “Gen 2”). The presence of an RFID tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored wirelessly by an “RFID reader”, also known as a “reader-interrogator”, “interrogator”, or simply “reader.” Readers typically have one or more antennas for transmitting radio frequency signals to RFID tags and receiving responses from them. An RFID tag within range of a reader-transmitted signal responds with a signal including a unique identifier.
With the maturation of RFID technology, efficient communication between tags and readers has become a key enabler in supply chain management, especially in manufacturing, shipping, and retail industries, as well as in building security installations, healthcare facilities, libraries, airports, warehouses etc.
RFID tags are small. Integrating an antenna into a RFID tag is difficult at best. Design compromises necessary for available space result in antenna arrangements that are sub-optimal. They do not have a good low VSWR characteristic across their intended band of operation. When size and space are not issues, an antenna can be matched by loading it with reactive elements that may be tunable in order to compensate for a sub-optimal antenna. By properly selecting the size and placement of reactive elements, an antenna can be made to appear full size and have a low VSWR at an operating frequency. Such antenna compensation becomes more difficult when there is little space to work with.
The problem of antenna compensation becomes even more difficult in an RFID tag environment. An RFID tag antenna receives interrogation signals from an RFID interrogator (also known as a “reader”). A typical passive RFID tag responds by backscattering part of the received signal by controlling its antenna to do so. Power for the RFID tag is obtained by allowing part of the energy received by the RFID tag antenna to feed a charge pump connected to the antenna. The process of pumping charge from the antenna affects its impedance and thus the characteristics of the antenna.